Big brands have suspended U.K. spending on YouTube and other assets owned by the Google parent amid reports of ads appearing on hate sites and next to videos from extremist groups.

Pivotal Research Group analyst Brian Wieser on Monday downgraded his rating on the shares of Google parent Alphabet, citing negative recent headlines in Britain.

As reported, last month it emerged that big-name advertisers, including car-maker Jaguar Land Rover and media and information firm Thomson Reuters, suspended or limited their U.K. digital advertising spending following a report in The Times, part of Rupert Murdoch's News Corp, that said that their ads have appeared on hate sites and next to videos on Google's YouTube from supporters of Islamic terrorists and other extremist groups.

And late last week, the U.K. arm of advertising agency giant Havas said it was halting advertising on Google and YouTube "until further notice." News Corp CEO Robert Thomson argued that advertisers "need to go back to basics to protect their brands from serious damage."

Wall Street is also starting to take note, with Wieser highlighting "brand safety concerns" and recent stock gains. "Google is facing a serious issue in the U.K. with brand safety issues, which has global repercussions," Wieser wrote in his Monday report. "Although spending by advertisers who have announced their intention to suspend spending on YouTube and other Google properties is relatively small so far, we think that awareness of the incident will marginally curtail global growth this year versus prior expectations, leading us to reduce our price target on Alphabet slightly, to $950 versus $970 previously."

That — along with the stock’s recent run-up — lead to the decision to downgrade Alphabet shares from "buy" to "hold."

"In a year where ad quality and brand safety has possibly become the biggest issue facing the advertising industry, last week’s news that agency holding company Havas, with the sixth-largest global media network, is pulling its spending from Google’s YouTube and Google Display Network in the U.K. because Google was 'unable to provide specific reassurances, policy and guarantees that their video or display content is classified either quickly enough or with the correct filters' is a big deal," Wieser argued.

According to latest press reports, other large brands working with agencies beyond Havas, including the U.K. government, L’Oreal, HSBC,Sky, Marks & Spencer, McDonald’s and Audi, have also signaled that they would or could suspend their advertising on YouTube and other Google ad products.

Google has said it would focus on improving policies regarding what is deemed "safe" for advertisers, including a refinement of definitions of hate speech and inflammatory content, strengthening controls and accelerating takedowns of content.

"The approach comes across to us as attempting to minimize the problem rather than eliminating it, which is the standard we think that many large brand advertisers expect," Wieser said.

Liberum Capital analyst Ian Whittaker has said that U.K. TV giant ITV could benefit from the digital ad controversy. "News that the U.K. Cabinet Office has withdrawn government adverts from YouTube after its adverts were shown alongside hate material is another sign of increasing concerns from advertisers about the potential damage to their brands from online advertising," he wrote. "We see this as benefiting traditional media, including ITV, as advertisers push spending back towards more 'trusted' channels [that] can also offer 'safe' online video content of their own."